We divided each semester's students into four groups of approximately the same size based on the students' cumulative GPAs, and we used independent-samples t tests to compare the lowest GPA group in the control section, spring 2012, with the corresponding GPA group in each of the treatment sections, fall 2012 and spring 2013, in terms of mean scores on the ACS exam. The results are displayed in Figs. 3 and 4